This invention relates generally to spin valves and tunnel valves used as magnetic field sensors. More particularly, it relates to spin valves with an improved magnetic pinning layer.
Conventional magnetoresistive (MR) sensors, such as those used in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin of the GMR effect is that the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure, called a spin valve, comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metal layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an antiferromagnetic layer which exchange-couples to the pinned layer. The unpinned layer or free ferromagnetic layer is free to rotate in the present of any small external magnetic field.
Spin valve structures have been identified in which the resistance between two uncoupled ferromagnetic layers is observed to vary as cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow. The spin valve produces a magnetoresistance that, for selected combinations of materials, is greater in magnitude than AMR. In general, the larger xcex94R/R is the better the spin valve""s performance.
Spin valve (GMR) read heads require two main improvements for future high density recording needs, which are larger signal for detecting ever smaller magnetic bits and smaller read gaps requiring thinner pinning layers. Most previously described spin valve use NiO as the antiferromagnetic or pinning layer deposited adjacent to the pinned layer for exchange-coupling to fix or pin the magnetization of the pinned layer. Through exchange anisotropy with the NiO antiferromagnetic layer, the magnetization of the pinned layer is held rigid against small field excitations, such as those that occur from the signal field to be sensed. However, the low magnetic anisotropy energy for NiO, less than 105 erg/cm2, led to a weak pinning field and a high critical thickness, greater than 400 xc3x85, for pinning layer. Additionally, the low ordering temperature of 250xc2x0 C. led to thermally unstable pinning.
U.S. Pat. No. 5,665,465 issued Sep. 9, 1997 to Gyorgy et al. discloses an article including a magnetically hard oxide layer in contact with a magnetically soft oxide layer, with spins in the latter at room temperature exchange-coupled to the oriented spins in the former. Both hard oxide layer and soft oxide layer consist of ferrimagnetic spinel-type oxides. However, Gyorgy et al. only teaches the existence of exchange anisotropy between two magnetic metal oxide layers, which are only in expitaxial crystalline structures. Gryorgy does not teach about the exchange anisotropy between a magnetic metal oxide layer and a metallic layer. Furthermore, Grygory does not teach or suggest the use of the exchange-coupled structures in spin valves.
An article entitled xe2x80x9cEnhanced Blocking Temperature in NiO Spin Valves: Role of Cubic Spinel Ferrite Layer Between Pinned Layer and NiOxe2x80x9d by R. F. C. Farrow et al. submitted to Applied Physics Letters on Jun. 19, 2000 discloses simple spin valves including an interfacial oxidized Fe layer inserted at the pinned layer/antiferromagnetic NiO layer interface to increase blocking temperature and pinning field for spin valves. The Fe-oxide layer is converted to a cubic spinel ferrite (Ni0.8Fe2.2O4) layer by solid state reaction with the NiO. Unfortunately, spin valves produced by this technique do not give larger signals for detecting smaller magnetic bits and do not have a small read gap.
There is a need, therefore, for an improved spin valve including pinning layers that overcome the above difficulties.
Accordingly, it is a primary object of the present invention to provide spin valves having an exchange anisotropy between a magnetically hard ferrite layer with poly-crystalline structure exchange-coupled to a magnetic metal pinned layer.
It is another object of the present invention to provide a spin valve with larger signal for detecting smaller magnetic bits.
It is a further object of the invention to provide spin valves with smaller read gaps.
It is an additional object of the invention to provide spin valves including highly corrosion-resistant pinning layers.
It is another object of the present invention to provide high efficiency spin valves, in which current is not shunted through the pinning layer.
It is a further object of the present invention to provide a method of producing spin valves having above properties.
These objects and advantages are attained by an exchange-coupled magnetic structure having cobalt-ferrite pinning layers exchange-coupling with adjacent ferromagnetic metal layers. Cobalt-ferrite is known to be a ferrimagnetic insulator with high magnetic anisotropy and high Curie temperature. In addition, cobalt-ferrite is a stable oxide phase highly resistant to corrosion.
A conventional magnetoresistive sensor, such as a spin valve or a tunnel junction valve, includes an antiferromagnetic layer, a ferromagnetic pinned layer having a magnetization pinned by the antiferromagnetic layer, a ferromagnetic free layer with a magnetization independent of the ferromagnetic pinned layer, and a spacer layer between the ferromagnetic pinned layer and the ferromagnetic free layer, which is a conductive spacer layer for a spin valve and is an insulating barrier layer for a tunnel junction valve.
According to a first embodiment of the present invention, a cobalt-ferrite pinning layer is used in place of the antiferromagnetic layer. Since the cobalt-ferrite layer has a magnetic moment (unlike an antiferromagnetic pinning layer) the design must take this magnetic moment into account. Since a reduced pinning layer thickness is highly desirable, there is no inherent problem with the pinning layer contributing magnetic moment.
According to a second embodiment of the present invention, the cobalt-ferrite layer is used in an AP-pinned spin valve design which is similar in structure to the magnetoresistive sensor of the first embodiment except that the pinned layer is replaced by an AP pinned structure. An AP pinned structure typically includes a first and a second magnetic layer and an AP spacer layer disposed between the first and second magnetic layers. The first magnetic layer located adjacent to the spacer layer than the second magnetic layer is made of cobalt-ferrite. The net moment of the AP pinned structure is reduced and the coupling field is increased relative to the pinned layer of the first embodiment since the second magnetic layer couples antiparallel to the cobalt-ferrite layer.
For most AP pinned spin valves, the AP spacer layer is typically made of Ru (although other paramagnetic metals, e.g. Cu, may be suitable). The cobalt-ferrite layer may require a more complex structure: ferrite/magnetic metal coupling structure including a cobalt-ferrite layer contacting a magnetic metal layer, to obtain the best results. This structure is used in either or both first and second magnetic layers of an AP pinned spin valve according a third embodiment of the present invention. Since the cobalt-ferrite layer has a magnetic moment, the magnetic metal layer may be very thin. Typically, 5 xc3x85 thick is sufficient to enhance AP coupling.
A layer of magnetically soft ferrite material, such as manganese zinc ferrite, with a ferrite/magnetic metal coupling structure may also be used in an AP-free structure design according to a fourth embodiment of the present invention. An AP-free magnetoresistive sensor has a basic structure similar to the magnetoresistive sensor described in the first embodiment except that it utilizes an AP-free structure. An AP-free structure typically includes a first magnetic layer, a second magnetic layer, and an AP spacer layer between the first and the second magnetic layers. The first magnetic layer includes a first soft ferrite layer contacting a first magnetic metal layer adjacent to the AP spacer layer. The second magnetic layer is made of a soft ferromagnetic metal. The soft ferrite layer is electrically non-conducting as far as a magnetoresistive sensor is concerned. The second magnetic layer is coupled through the AP spacer layer to the soft ferrite layer. This layer may be nanolayered with a ferromagnetic metal to improve the AP coupling for the AP free structure.
Alternatively, two soft ferrite/magnetic metal structures may be used for both the first and the second magnetic layers in an AP-free structure.
A cobalt-ferrite layer may be added to a magnetoresistive sensor, wherein an antiferromagnetic layer is between the cobalt-ferrite layer and the pinned layer, to assist in biasing according to a fifth embodiment of the present invention. The antiferromagnetic layer pins both the cobalt-ferrite layer and the pinned layer. The magnetic moment of the cobalt-ferrite layer is parallel or unparallel to the magnetic moment of the pinned layer depending on the differences in the coercivities of these two layers.
According to a sixth embodiment of the present invention, cobalt-ferrite layers of magnetoresitive sensor, such as spin valves, are made by reactive co-sputtering of Co and Fe in an oxygen/argon gas mixture. Two underlayers of Cr and RuAl are sputter-deposited on a glass substrate. A cobalt-ferrite layer is deposited onto these underlayers by reactive co-sputtering of Co and Fe. These underlayers are used to ensure the glass substrate temperature reached 250xc2x0 C. during the cobalt-ferrite layer growth. Following the deposition of the cobalt-ferrite layer, the glass substrate is cut into several pieces, which are loaded into an ultra high vacuum evaporation system for the depositions of the pinned layer, spacer layer and the free layer. The surface of the cobalt-ferrite layer is cleaned by exposure to a beam of oxygen atoms to remove impurities (adsorbed during transfer) prior to deposition of the pinned layer. During the deposition of the pinned layer the substrate temperature is held at about 2000xc2x0 C. A spin valve made by this method typically has a very small signal, about 0.3%, due to several factors including the material of the spacer layer, the presence of the metallic Cr/RuAl underlayers and the non-optimal thickness of the pinned layer. Nevertheless, the pinning field is achieved about 60 Oe, and the blocking temperature is greater than 200xc2x0 C.
The cobalt-ferrite layer may be formed by reactive sputtering of CoFe2 target in oxygen/argon gas mixture according to a seventh embodiment of the present invention. The cobalt-ferrite layers are deposited onto a glass substrate by reactive sputtering of CoFe2 target in an oxygen/argon gas mixture with the substrate temperature held at room temperature during the film growth. Following the deposition of the cobalt-ferrite layer, the pinned layer, the spacer layer, and the free layer are also deposited at room temperature by sputtering. Spin valves made by this method have very substantial magnetoresistance values in spite of the fact that the substrate temperature is not optimized. A wide range of oxygen conditions is used during cobalt-ferrite layer formation.
Alternatively, the cobalt-ferrite layer may be formed by evaporation of Co and Fe from a Co/Fe alloy source (containing 40% Fe) along with a flux of oxygen atoms from a RF oxygen atom beam source onto Si, SiO2 and single crystal MgO (001) substrates with the substrate temperature held at 250xc2x0 C.
Analogous methods may be used to deposit the magnetically soft ferrite layer in an AP-free spin valve.
Magnetoresistive sensors having cobalt-ferrite layers as described in above embodiments are incorporated in a magnetoresistive (MR) read write head according to an eighth embodiment of the present invention. A MR read/write head includes a first shield contacting a first gap, a second shied contacting a second gap, and a spin valve disposed between the first and second gaps.
MR read/write heads of the type described in the eighth embodiment are incorporated in a disk drive according to a ninth embodiment of the present invention. A disk drive includes a magnetic recording disk connected to a motor and a MR read/write head connected to an actuator. The motor spins the magnetic recording disk with respect to the MR read/write head, and the actuator positions the GMR head relative to the magnetic recording disk.